Closing your eyes is probably the easiest way to avoid seeing things you don’t want to see. But if you face the sun (with your eyes still closed), you will still see a red glow telling you that some light is still transmitted. And seeing a glow rather than any coherent image tells you that any light that manages to transmit to your eyes has been scattered, leaving the objects on the other side of your eye unidentifiable.

But new technology developed in Caltech’s Biophotonics Lab might be able to reconstruct those red glowing images on the other side of the eyelid, or any other piece of biological tissue.

The experiments are a proof-of-principle study that demonstrates the possibility of the playback of light scattered in biological tissues to restructure the original light beam, report Caltech bioengineering professor Changhuei Yang and postdoc Zahid Yaqoob in last week’s Nature.

In their published study, the researchers used a slab of chicken breast, which has scattering properties similar to those of human tissue. The major complication was the precise alignment of the set up involving a laser, several filters, polarizers, beam splitters, wave plates, mirrors, a detector, and a hologram recording medium.

When light bounces off of objects, a complicated pattern arises, much like how the crashing of waves in water against a collection of moored boats results in a complicated pattern. This pattern is useful because it contains information about the objects themselves. Using the capabilities of a holographic recording medium, one can record on a photographic plate the distorted light field pattern that is transmitted through the biological tissue. The result is a complete recording of the light field pattern.

By illuminating the recorded hologram with an appropriately position readout laser beam, the distorted light field pattern can then be made to travel back through the tissue in a time reversed fashion. If all goes well, the photons completely retrace their paths and reconstruct the original beam that illuminated the tissue.

Because biological tissues are much more scattering than what has been tried, recording the pattern of light scattered from tissues was thought to be too complex to be possible. The idea, which first came to Prof. Yang in his PhD years at MIT, started out as what Prof. Yang calls a “high risk attempt” to “try out something interesting and new.” The project has now bloomed to become a permanent portion of the present lab.

The research, according to third year graduate student Emily McDowell, has wide applications when considering the pattern of scattered light reflected rather than transmitted. Ranging from neuron activation to removal of an embarrassing port-wine stain, applications, as suggested by Prof. Yang, involve basically any situation where you want to concentrate light on strong scatterers in tissue. A potential form of photodynamic cancer therapy based on this new technology, for instance, would employ drugs that are photoactivatable and strongly scattering; or the use of photovoltaics charged using their new technique allow the removal of bulky power sources from implants, such as pacemakers.